RAID questions/advise

[Peter Sabaini]

On Wednesday 24 September 2008 18:59:57 Michael S. Peek wrote:
> Hi guys,
>
> I've had a couple of RAID boxes ticking away in the corner for years now
> without a problem.  But now our needs have expanded, and I'm looking to
> build replacements.  Big replacements.  And I consider myself to be
> anything but an expert in the field, especially where mdadm is
> concerned.  So I have a few questions to ask in hopes that someone out
> there can help me out.
>
> How large of an array can mdadm handle?

That's something I'd also like to know. Strangely enough, nothing in the 
documentation mentions any size limit. Maybe there are none, or maybe they 
are embarrassingly low :-)

> If I use my hardware RAID cards in JBOD mode, how does the kernel handle
> naming drives when there's more than 26 drives on the system?  (i.e.
> what does it do when it reaches /dev/sdz and there are drives left to be
> named?)
>
>  From what I hear, ext3 can handle filesystems up to 32TB in size, but
> has anyone actually done this?  Can anyone attest to how well it works?
> Or is there another filesystem type that's better suited to large (12TB
> - 32TB) filesystems?

FWIW, JFS supports up to 4 Petabyte; I also like its robustness

peter.

> Finally, according to the mdadm FAQ, when a drive goes down:
> > 19. What should I do if a disk fails?
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   Replace it as soon as possible:
> >
> >     mdadm --remove /dev/md0 /dev/sda1
> >     halt
> >     <replace disk and start the machine>
> >     mdadm --add /dev/md0 /dev/sda1
>
> Since my OS drive will not be a part of the RAID (it'll have a mirrored
> RAID of it's own), I presume that the halt command won't be necessary.
> I assume that it would be perfectly reasonable of me to remove the drive
> and replace it while the system is running?  I.e. mdadm can handle
> running in degraded mode for the duration of the replacement/rebuild
> process?  (This is a deal-breaker question -- if mdadm can't, then I'll
> have to pursue other measures.  Hotswap drives will be up and running at
> all times though, so I presume I can configure mdadm to make use of them
> immediately upon detecting a drive failure.)
>
> My other option is to get a SAN/NAS of some type, but building machines
> like this has proven to be very effective and cheap compared to
> SAN/NAS'es, at least in the past.  I haven't checked recently though,
> but any advice is welcome.
>
> Pondering my options,
>
> Michael


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